Auxin is a class of plant hormones and involved in various phases such as development, growth and environmental responses of plants. The substance most ubiquitously present as a natural auxin is indole acetic acid (IAA) and natural auxins such as indolebutyric acid (IBA) and 4-chloroindoleacetic acid are also known. In contrast, as synthetic auxins, p-chlorophenoxyacetic acid, 2,4-dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-chlorophenoxybutyric acid (MCPB) and the like are known.
IAA, a natural auxin, is instable. In addition, a plant has a decomposition pathway thereof within the body. Therefore, a synthetic auxin is generally used for agriculture. For example, p-chlorophenoxyacetic acid is used as a fruit set accelerator for tomatoes and eggplants. Furthermore, 2,4-D is used as a herbicide and an agent for culturing a plant tissue, and MCPB is a selective herbicide used in rice paddies.
Auxins are biologically synthesized through complicated pathways. To be more specific, the presence of two pathways through or not through L-tryptophan has been confirmed. The pathway through L-tryptophan is further branched in 4 or more pathways, which are separately catalyzed by different enzymes (FIG. 1). Up to present, as substances inhibiting biosynthesis of auxin, L-α-(2-aminoethoxyvinyl)glycine (AVG), L-aminooxyphenylpropionic acid (L-AOPP), aminooxyacetic acid (AOA) and 2-aminooxyisobutyric acid (AOIBA) are known (Patent Literature 1).